Wave-signal translating system



- designed Fatented Fete. d, 194? STATES WAVE- SIGNAL SLA'I'ING SYSTEM Original application December 31, 1943, Serial No. 516,394. Divided and this application October '10, 1944, Serial No. 557,953

3 Claims. (CL 177-380) This invention relates, in general, to wave-signal translating systems and is particularly directed to coding or secrecy aspects of transponder systems. As used throughout this specification and in the appended claims, the term transponder system" is intended to describe a wavesignal translating system which includes a receiving system and a transmitting system so arranged that a predetermined answer or reply signal is transmitted in' response to a received interrogating signal, the reply and interrogating signals preferably comprising radiant-energy wave signals.

This application constitutes a division of copending application Serial No. 516,394, filed December 31, 1943, in the name of Bernard D. Loughlin and assigned to the same assignee as the present invention.

Transponder systems of the type under consideration are frequently utilized as beacons for supplying direction-finding information to air traffic. In the usual installation the system is to translate pulse-modulated wave signalsin a pulse-by-pulse fashion. That is, the receiving system is designed to receive a radiated interrogating signal having pulse-type characteristic variations, such as pulse-type amplitude variations, and the transmitting system is designed to transmit a reply signal having similar characteristic variations, their operation being such that each modulation pulse of the received interrogating signal causes the transmission of a single pulse of the reply signal. It has been proposed that the reply signal be coded to render the system secret and thus preclude unauthorized listeners from deriving useful information. The arrangements employed to accomplish this coding have consisted oi mechanical devices for so interrupting the normal pulse-by-pulse operation of the transponder system that the transmitted pulses of the reply signal appear in spaced groups in accordance with a prearranged coding schedtie.

Wi'iile the described coding arrangements have proved satisfactory, they require that the interrogating signal be continuously received by the transponder system for a considerable period of time to insure the transmission of a complete coded answer. In some installations, this time requirement may be an undesirable'limitation.

This is especially true where the complete coded reply must he transmittted in the shortest possible period of time. 7

Furthermore, in certain installations it is desirable that a complete coded reply signal be transmitted in response to a single received pulse of an interrogating signal. The prior art arrangements referred to above do not lend themselves particularly well to such installations.

It is an object of the invention, therefore, to provide an improved wave-signal transponder system which avoids one or more of the abovementioned limitations of prior art arrangements.

It is a further object of the invention to provide an improved transponder system for transmitting a coded reply'signal in response to a received interrogating signal.

i It is a further object of the invention to provide a wave-signal transponder system which includes improved coding arrangements in its transmitting system.

In accordance with one feature of the invention, the transponder system includes a wavesignal translating system for translating a signal pulse. This translating system comprises .an input circuit for receiving the signal pulse and means coupled to the input circuit for deriving from the received signal pulse a plurality of pairs of signal pulses occurring in a predetermined time sequence and individually having a predetermined time separation. The system further includes means responsive to the plurality of pairs of signal pulses for deriving a pulse-modulated output signal comprising components having pulse durations determined by the time separa tlon of the individual signals of each of the aioresaid pairs and pulse spacings determined by the aforesaid time sequence. This output signal is suitable for controlling a transmitter included in the transponder arrangement to transmit a coded reply signal of similar wave form.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following de scription taken in connection with the eccom .panying drawings, and its scope will be pointed out in, the appended claims.

Fig. 1 of the drawings is a schematic circuit diagram of a wave-signal transponder system in cluding an arrangement, in accordance with the invention, for transmitting a coded reply signal in response to a received interrogating signal; Figs. 2 and 3 comprise graphs utilized in explaining the operation of the errangement'oi Fig. 1; Fig. 4 represents a' portion of the Fig. l. arrangement in modified form; while Fig. 5 com prises a set of graphs utilized in explaining the operation of the Fig. 4 arrangement.

Referring now more particularly to the draw" lngs, there is represented in Fig. 1 a wave-signal transponder system having improved secrecy features in accordance with the present invenanasac tlon, For convenience of explanation, the system may be considered as a radio beacon adapted to transmit direction-finding information to interrogating aircraft. As represented, the radio beacon includes a selective receiving system which forms the subject matter of the above-identified 'copendin'g application. This selective receiving tude limiter I3, a delay-network unit to be described more particularly hereinafter, aclipping amplifier l6, and a keyed multi-vibrator 15. The band-pass selectors of receiver l2 and the amplitude limiter l3 are arranged, respectively, to limit the frequency components and amplitude of the signal applied to unit 20 in order that this signal may have a predetermined wave form, that is,

a predetermined maximum amplitude and predetermined slopes at the leading and trailing edges of its pulse type amplitude variations. The bea con further includes a transmitting or wave-signal translating system coupled to the output circuit of multivibrator l5. This transmitting system consists of a delay-network unit ill, described more particularly hereinafter, an amplifier it, a carrier-frequency signal generator and modulator or transmitter H, and a transmitting antenna I8, [9. Transmitter I1 is normally in a quiescent condition but is energized or controlled by control signals translated through amplifier l6. Elements |ll-I9, inclusive, may be of any conventional construction and operation; By Way of illustration, amplitude limiter l3 may be similar to the limiter arrangement designated lb of United States Letters Patent 2,271,203, granted January 27, 1942, in the name of Jasper J. Okrent and assigned to the same assignee as the present invention. Clipping amplifier I l may constitute an mplifier biased to pass only signal components having an amplitude value exceeding a critical level, as illustrated at page 372, Fig. 2173 of Principles of Television Engineering, by Donald G. Fink, published in 1940 by McGraw-Hill Book Company, Inc. Multivibrator it may be a trigger circuit of the type shown at page 176, Figs. 4-9 of Ultra-High-Frequency Techniques, by J. G, Brainerd et al., published in 1942 by D. Van Nostrand Company, Inc.

Neglecting for a moment units Eli and all and considering element It to be a conventional amplifier, it will be readily apparent to those skilled in the art that the arrangement of Fig. 3. comprises a conventional radio beacon. Since the operation of such apparatus is generally well understood, a detailed description and explanation thereof is unnecessary. Briefly, however,

an aircraft desiring to obtain information from the beacon sends out an interrogating signal having pulse-type amplitude variations of equal pulse durations and equal pulse spacings, This signal is intercepted by antenna system it, ii and translated by receiver E2, the modulation components of the signal appearing in the output circuit thereof as a unidirectional signal having amplitude variations corresponding to those of the received signal. This signal is applied by way of elements l3 and it to keyed multivibra tor i5, which generates a single control pulse of rectangular wave form and of any desired. pulse duration as determined by the circuit pasystem, when interrogated by an inquiring air-,

tive units, as illustrated.

'IKE

drameters of the multivibrator, one such control pulse being generated thereby in response to each modulation pulse of the interrogating signal. The control pulses from multivibrator 15 are translated through amplifier l6, thereby to control transmitter ll to transmit in response to each such control pulse a single pulse of an answer or reply signal, the transmitted pulses having substantially the same pulse duration, spacings and wave form as those translated through amplifier it. Thus, the described transponder craft, transmits a reply signal having pulse-type amplitude variationswhich individually correspond to modulation pulses'of the interrogating signal. Expressed differently, the beacon functions in a pu1se-by-pulse fashion-to transmit an answer in response to an interrogating signal.

Referring now more particularly to the delaynetwork units 20 and 40 of the Fig.1 arrange? ment, these units are associated, respectively, with the receiver and transmitter portions of the transponder system, being provided to afford the desired secrecy characteristics. Unit 20 is a transversal time-delay filter network having input terminals 2i, 2! and output terminals 22, 22. The delay network comprises a transmission line consisting of a pair of spaced parallel conductors 23, 23, coupled at one end to input terminals 2 l, 2 l and terminated at the opposite end in the surge or characteristic impedance of the line designated 2%. The electrical characteristics of transmission line 23, 23, are preferably such as to effect a uniform predetermined time delay per unit oi line length to a signal translated thereby. The physical length of the line is determined by the degree of secrecy desired, as

will presently be explained. A plurality of adjustable taps are associated with the line so as to be freely movable therealong, three such taps 25, 26 and 2'! being shown in the drawings. Each of taps 25, 26 and 271' is includedin circuit with an individual one of isolating resistors 28, 29 and till and a common impedance 3| is coupled between output terminals 22, 22, of unit 20. Each isolating resistor has a relatively high impedance as compared with that of common impedance 3|.

Unit at likewise comprises a transversal timedelay filter network having input terminals 4|, ii, output terminals t2, a parallel-conductor transmission line all, 43, coupled at one end to terminals ll, ti and terminated at the opposite end in its surge or characteristic impedance M. Transmission line d3, t3, may be of the same general construction and design as line 23, 23, of unit 20. Unit 4 9 also includes a plurality of adjustable taps it, 46 and 41 adiustably connected to transmission line 43, d3, isolating resistors all, 49 and 60 individually in circuit with each of the taps, and a common output impedance 55 having a low value of impedance in comparison with that of the individual isolating resingle tap is effective in each of such units, namely, tap 25 of unit 28 and tap 45 of unit 30. It will be further assumed that these taps are positioned adjacent to the input terminals of their respec- For the assumed conditlon, a received interrogating signal is translated directly from wave-signal translator l2 through unit i3, tap T25 of unit 20 and amplifier i l to inultivilorator it. Also, the control pulses generated in multivibrator i are translated directly therefrom through tap 45 of unit 48 to amplifier it and transmitter 17. Accordingly, for these conditions the operation of the trans.- ponder system is substantially as described.

Now it will be assumed that all of the taps of units it and iii are effective and that such taps are positioned along their associated transmission lines in the manner indicated in the drawings. It will be further assumed that clipping amplifier M is adjusted to have a clipping level indicated by broken-line curve 0 of Fig. 2 and that the interrogating signal as applied to input terminals M, M, of unit 20 has the wave form represented by curve 625, of Fig. 2. From this last-mentioned curve, it will appear that the interrogating signal as applied to terminals 2|, 2|, has a maximum amplitude limited to a predetermined value c by unit I3 and a group of three pulse-type amplitude variations of equal pulse durations t occurring in a predetermined time sequence represented by the unequal pulse spacings ti and 232. It will be understood that'the interrogating signal may comprise repeating groups of pulse-type amplitude variations individually having pulse durations and spacings as described and a time separation which is long with reference to the period of a single group. For convenience of explanation, however, the interrogatmg signal will be considered as consisting of but a single group of such amplitude variations.

The response of the transponder system to such a received signal is illustrated by the curves of Figs. 2 and 3 in which the subscripts indicate the elements where the represented signal voltages are derived. Signal voltages are derived at each of taps l5, 2t and 21. If the signal attenuation along line 23, 23 be neglected, the derived signal voltages individually have a predetermined maximum amplitude equal to the above-mentioned value c and pulse-type amplitude variations corresponding to the modulation of the received interrogating signal. Due to the timedelay characteristics of transmission line 23, 23, and the relative spacings of the adjustable taps, these signal voltages have a time relation with reference to each other such that predetermined amplitude variations of the several signal voltages occur substantially in time-phase relation. More specifically, the third amplitude variation of signal are, the second amplitude variation of signal 828, and the first amplitude variation of signal (221 occur in time-phase relation. These signals add up across load impedance 3i, producing an output signal at terminals 22, 22, as

represented by curve 822, having one amplitude variation or pulse of such magnitude as to exceed the clipping level 0 of clipping amplifier Hi. This amplitude variation of the output signal corresponds in time with the above-described inphase components of. the individual signals obtained at taps it and El and produces a control effect which keys multivibrator H5.

The inultivibrator generates and applies to input terminals ll, ll, of unit it a single unidirectional pulse of substantially rectangular wave term. The time-delay characteristics of transmission line cause taps t5, t6 and t? to derive a plurality of signals individually having a wave form corresponding to that of the applied pulse and a time relation with reference to each other such that the signals derived at the individual tapping points app ar in a predetermined time sequence. These signals develop across the common impedance 5| a control signal, represented by curve on of Fig. 3, for application to amplifier 16. This control signal likewise has pulse-type amplitude variations of equal pulse durations t4 determined by the period of multivibrator l5 and unequal pulse spacings t5 and ts determined by the relative positions of taps 45, 46 and 41. The reply signal transmitted from antenna system I8, I9 in response to this control signal is a pulse-modulated signal comprising components having pulse durations and pulse spacings corresponding to those of control signal 842. In other words, the transponder system including units 20 and 40 with taps arranged as indicated in the drawings responds to interrogating signals having pulse-type amplitude variations occurring in the order mentioned and transmits in reply thereto a complete coded answer, coded in accordance with the relative settings of the taps included in unit 40.

From the foregoing description, it will be apparent that the arrangement of Fig. 1 comprises a wave-signal transponder system for transmitting a coded reply signal in response to an interrogating signal having characteristic variations appearing in a predetermined time sequence. With reference to the particular coded reply signal transmitted, unit 40 comprises means for deriving from the interrogating signal a plurality of signals individually having predetermined characteristic variations and a time relation with reference to each other such that predetermined variations of the individual characteristics of the plurality of signals appear in a predetermined time sequence, which may be the same or difierent from the time sequence of the interrogating sigcharacteristic variations of the cal. Furthermore, amplifier I6 and wave-signal generator and modulator .l'l comprise means responsive to the predetermined variations of this plurality of signals for transmitting a coded reply signal having components corresponding in number to this plurality of signals and a spacing determined by the time sequence of such plurality of signals.

A modification of the coding arrangement for the transmitter portion of the transponder system is represented in Fig. 4 and indicated generally as 40'. This arrangement is similar in construction and design to unit 40, described above, and corresponding components thereof are designated by like reference numerals primed. However, unit 40' includes a second series of adjustable taps 45", $6" and 41" as well as isolating resistors d8", 49", and 50" and a common impedance 5|". The input circuit of a phase inverter 19, such as a triode amplifier with unity gain, is coupled across impedance 5!" and the signal derived in the output circuit thereof is applied to the input termnals of an integrating circuit H along with the signal voltage developed across common impedance 5|. The integrating circuit may be of the type shown at page 163,

Fig. 95 of Principles of Television Engineering. The output terminals A2, 42' of unit t0 are coupled to the output terminals of integrating circult H.

To utilize the described modification or the coding arrangement in the transponder system of Fig. l, multivibrator I5 is adjusted to generate a sharp pulse of rectangular wave form having a pulse duration 1:15, curve f of Fig. 5. This curve demonstrates the time relation of the pulses derived at the individual taps of the transaerasee versal time-delay filter network in response to a single pulse applied from multivibrator l5.

Phase inverter ID, by inverting the phase of the signals developed at taps 45", 46" and 41C, causes the signal delivered to integrating circuit H to have a wave form as represented by curve g. It will be noted that this signal efiectively comprises a plurality of pairs of signals appearing in a predetermined time sequence and individually comprising unidirectional signals of opposite polarities having a predetermined time separation. That is, for the assumed relative positions of the adjustable taps, the derived pairs of signals have individual time separations indicated he, tm and till and appear in a time sequence represented by intervals tl9, and 1520. The integrated signal obtained at output terminals 42,, 42', has a wave form as indicated by curve It and, after translation through amplifier I 6, controls wave-signal generator and modulator I! to transmit a reply signal having a modulation envelope of similar wave form. Thus, unit 40' permits the coded reply signal to have components of predetermined pulse duration and pulse spacings as determined by the selective positioning of the adjustable taps associated with transmission line 43', 43'.

The described coding arrangements are extremely flexible and provide a degree of secrecy which increases with the complexities of the particular arrangement utilized. The secrecy features of the transmitter portion may be varied by including any desired number of taps or pairs of taps in the coding arrangement of units 60 and M, respectively, spaced in accordance with any desired coding schedule. Likewise, the described coding arrangement of the selective receiver may be suitably modified to obtain varying degrees of secrecy, as particularly mentioned in the above-indentifled copending application.

Although it is preferred that coding arrangements be associated with both the receiver and transmitter portions of the transponder system, either one may be omitted. Where the coding feature is utilized in the transmitter but omitted from the receiver, a complete coded answer is transmitted in response to a single pulse modulation of the interrogating signal.

In any case, where coding of the transmitted reply signal is employed, it is desirable to provide a holding .or blocking feature in the receiver to disable the receiver for such a period of time following the tripping of multivibrator l that a complete coded reply may be transmitted before an interrogating signal is able to trip the multivibrator a second time. In many applications this feature will be inherent in the receiver since the transmitted signal; which is intercepted by the receiving antenna system H], H, in most installations will have such power as to overload the receiver circuits and disable the receiver for a period of time determined by its inherent time constant.

The described transversal time-delay filter networks are especially useful in the coding arrangements under consideration since they permitthe coding schedules to be altered through the simple expedient of adjusting the number or relative positions of the adjustable taps. The

maximum time delay desired to be obtained in the network governs the lengths of the transmission lines of the described networks.

In the specific modifications of the invention described above, the interrogating signal has been assumed to have amplitude variations appearing in a predetermined sequence. The invention is equally applicable to transponder systems designed to receive interrogating signals having pulse-type frequency or phase variations appearing in a predetermined time sequence. Therefore, the expression an interrogating signal having predetermined characteristic variations appearing in a predetermined time sequence, as used herein, is intended to include all such applications.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A wave-signal translating system for translating a signal pulse comprising, an input circuit for receiving said signal pulse, means coupled to said input circuit for deriving from said received signal pulse a plurality of pairs of signal pulses occurring in a predetermined time sequence and individually having a predetermined time separation, and means responsive to said plurality of pairs of signal pulses for deriving a pulse-modulated output signal comprising components havin pulse durations determined by said time separation of the individual signal pulses of each of said pairs and pulse spacings determined by said time sequence.

2. A wave signal translating system for translating a signal pulse comprising, an input circuit for receiving said signal pulse, means coupled to said input circuit for deriving from said received signal pulse a plurality of pairs of signal pulses occurring in a predetermined time sequence and individually having a predetermined time separation, and means including an integrating circuit responsive to said plurality of pairs of signal pulses for deriving a pulse-modulated output signal comprising components having pulse durations determined by said time separation of the individual signal pulses of each of said pairs and pulse spacings determined by said time sequence.

3. A wave-signal translating system for translating a signal pulse comprising, an input circuit for receiving said signal pulse, means coupled to said input circuit for deriving from said received signal pulse a plurality of pairs of signal pulses occurring in a predetermined time sequence and individually comprising unidirectional signal pulses .of opposite polarities having a predetermined time separation, and means responsive to said plurality of pairs of signal pulses for deriving a pulse-modulated output signal comprising components having pulse durations determined by said time separation of the individual signal pulses of each of said pairs and pulse spacings determined by said time-sequence.

BERNARD D. LOUGHLIN.

REFERENCES CITED The following references are of record in the file of thispatent:

UNITED STATES PATENTS Number Name Date 2,229,249 Lewis Jan. 21, 1941 2,266,401 Reeves Dec. 16, 1941 2,145,332 Bedford Jan. 31, 1939 2,188,970 Wilson Feb. 6, 1940 

